First Statistical Tests for Clumpy-Torus Models:Constraints from RXTE Monitoring of Seyfert AGN
Alex Markowitz,1 Mirko Krumpe,2 and Robert Nikutta3
1

University of California, San Diego, Center for Astrophysics and Space Sciences, MC 0424,
La Jolla, CA, 92093-0424, USA
2
Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany3
National Optical Astronomy Observatory, 950 N Cherry Ave., Tucson, AZ, 85719, USA
E-mail(AM): almarkowitz@ucsd.edu
Abstract
This presentation is a summary of highlights from Markowitz, Krumpe, & Nikutta (2014) and preliminary results from our follow-up paper, Nikutta, Krumpe, & Markowitz, in prep. We present an analysis ofmulti-timescale variability in line-of-sight X-ray absorbing gas as a function of optical classification in AGN
to derive the first statistical constraints for recent clumpy absorbing torus models. Such models represent
the paradigm shift away from the classical assumed solid donut morphology and towards a morphology
containing numerous discrete clouds or clumps. Such advances come courtesy of sustained long-term X-ray
monitoring; we use the vast archive of Rossi X-ray Timing Explorer (RXTE ) multi-timescale monitoringof dozens of type I and Compton-thin type II Seyfert AGN. We search for discrete absorption events due
to clouds transiting the line of sight; most of our twelve detected clouds are Compton-thin and are locatedin the outer BLR or inner dusty torus. We discuss the resulting implications for cloud distributions in thecontext of Clumpy torus models. We discuss cloud sizes, stability, and radial distribution across a wide
range of distances, and explore the exhibited range in density profiles for the highest-quality eclipse events.
We discuss possible connections to the mechanisms that form and launch clouds. In addition, all observed
clouds are sub-critical with respect to tidal disruption; self-gravity alone cannot contain them. External
forces, such as magnetic fields or ambient pressure, are needed to contain them. Otherwise, clouds must
be short-lived. Our results apply to both dusty and non-dusty clumpy media, and probe model parameter
space complementary to that for short-term eclipses observed with XMM-Newton, Suzaku, and Chandra.
Key words: galaxies: active – X-rays: galaxies – galaxies: Seyfert

1.

Introduction

The exact morphology of the circumnuclear gas in active
galactic nuclei (AGN) remains unclear, but holds the key
to answering how material gets funneled from radii of kpc
in the host galaxy down to the accretion disk, and ultimately feeds the supermassive black hole. This unsolved
question impacts our understanding of the eﬃciency of
black hole feeding as well as AGN duty cycles.
The community has long been in the process of shifting
away from quantifying emission and absorption processes
in Seyferts by modeling circumnuclear gas via a simplified homogeneous, Compton-thick donut morphology.
Instead, a new generation of models describe the torus
via distributions of numerous individual clouds (Elitzur
2007; Nenkova et al. 2008), although suggestions thatthe torus should consist of clouds go as far back as, e.g.,
Krolik & Begelman (1986); Krolik & Begelman (1988).

Often, the clouds are embedded in some outflowing windfrom the cold, thin accretion disk which feeds the black
hole. Observational support for these models so far hascome mainly from fitting IR SEDs in small samples of
Seyferts (Ramos Almeida et al. 2011; Alonso-Herrero etal. 2011).
Evidence for clumpy tori also comes from variations
in column densities NH of line-of-sight X-ray absorbing
columns in both (optically classified) type Is and IIs,
with timescales of variability ranging from hours to years(Risaliti, Elvis, & Nicastro 2002). The community has
also observed moderately Compton-thick variations in
NGC 1365 (Risaliti et al. 2007; Risaliti et al. 2009) andNGC 7582 (Bianchi et al. 2009), as well as rapid changes
in covering fractions of partial-covering absorbers (Puccetti et al. 2007; Risaliti et al. 2011; Sanfrutos et al.
2013). When adequate data exist, time-resolved spec-

troscopy of full eclipse events has yielded constraints
on density profiles in the transverse direction for longduration (3–6 months) eclipses in NGC 3227 (Lamer et
al. 2003) and Cen A (Rivers et al. 2011) and for eclipses∼1 d in NGC 1365 (Maiolino et al. 2010) and SWIFT
J2127.4+5654 (Sanfrutos et al. 2013). In addition, X-ray
obscuration is the only way to probe obscuration insidethe dust sublimation radius Rd , and to test the notionthat the dust-free BLR and dusty torus form a common
radially extended structure inside and outside Rd , resepctively (Netzer & Laor 1993; Elitzur 2007; Gaskell et
al. 2008).
Typically for clumpy-torus models such as the
Clumpy models by Nenkova et al. (2008), the cloud distribution is preferentailly concentrated towards the equatorial plane, and is described by a Gaussian angular distribution σ, mean number of clouds along an equatorial
ray N0 , and a given radial power-law distribution out tosome maximum radius, with each cloud being identicaland having the same optical depth.
A statistical survey of the environments around supermassive black holes has been needed to properly constrain parameter space in the Clumpy models. These
parameters so far have been constrained only from IR
SED fitting, as mentioned above. In our 2014 paper, Markowitz, Krumpe, & Nikutta (2014; hereafter
MKN14), we present the first such X-ray-absorptionbased survey, the longest AGN X-ray monitoring study
to date. Our survey quantifies line-of-sight X-ray absorption by clouds that transit the line of sight to thecentral engine. We assessed the relevance of Clumpy
torus models as a function of optical classification by
exploring absorption over a wide range of length scales(both inside and outside the dust sublimation zone).
2. Methodology
We consider archival data collected with RXTE ’s Proportional Counter Array (PCA). RXTE was the first Xray mission to operate using sustained monitoring campaigns, with regularly spaced visits, usually 12 ks each,over durations of weeks to years.
RXTE visited 153 AGN over its lifetime. We only
consider sources with mean 2–10 keV flux ≳ 8 × 10−12
erg cm−2 s−1 , and visited at least four times during themission. We do not consider blazars. Due to the PCA’s
energy resolution, we cannot accurately quantify changes
in NH when a steady full-covering Compton-thick absorber is present, other than verifying that none of theseven Compton-thick sources monitored by RXTE transitioned to/from being Compton-thin. The final target
list consists of 37 type I and 18 Compton-thin type II
AGN. Here, we classify “type I” to include Sy 1.0, 1.2,
and 1.5; we classify “type II” to include Sy 1.9 and 2(we had no type 1.8s in our sample). 11/18 type IIs

in our sample show evidence for “hidden” BLRs, manifested via scattered/polarized optical emission or via IR
Paschen lines, and we adhere to the assumption that
BLRs do exist in all our objects, and that optical classifications are due to increasing levels of obscuration in the
optical band, independent of assumptions about systemorientation.
We examined 7–10/2–4 keV hardness ratio (HR1)light curves; assuming full-covering absorption, HR1
peaks at column densities roughly 1 − 3 × 1023 cm−2
while giving us sensitivity down to ∼ a few 1022 cm−2 .
A simple power-law fit to the 2–10 keV band, not accounting for absorption, can also indicate excess absorption: measurements of the “apparent” photon index suddenly falling to values lower than ∼ 1.5 − 1.6, the typically flattest coronal power-laws measured in unabsorbed
Seyferts, are likely due to absorption events. We distinguish between “secure” vs “candidate” eclipse events:
the former group are those confirmed via binned timeresolved spectral fitting to have a temporary increase in
NH . Additional analysis details and eclipse identification
criteria are presented in MKN14.
As a caveat, we are only able to detect an absorption
event and rule out spectral pivoting if the cloud’s covering fraction is greater than ∼ 80 − 90%, and if its ionization parameter ξ [erg cm s−1 ] is less than log ξ ∼ 1 − 2.
Our findings are thus complementary to those derived
using the archival databases of other X-ray missions:
XMM-Newton, Chandra, and Suzaku can probe clouds
with lower column densities, partial covering, and/or
high-ionization absorption, and eclipse events with durations ≲1–3 d (Turner et al. 2008; Risaliti et al. 2011).
However, unlike RXTE, these missions generally do not
perform sustained monitoring campaigns and are notable to detect absorption events longer than ∼ a few
days. In addition, RXTE -PCA’s coverage above 10 keV
aﬀords increased sensitivity to higher absorbing columns.
3. Summary of main results
We find twelve “secure” X-ray absorption events (confirmed with spectral fitting) across eight Seyferts:
NGC 3783, NGC 3227, MR 2251–178, Mkn 79, Mkn 509,
Cen A, Mkn 348, and NGC 5506. Four of theseeclipse events were reported previously (Lamer et al.
2003; Rivers et al. 2011; Rothschild et al. 2011; Smithet al. 2001; Akylas et al. 2002). We also report an
additional four “candidate” events in three Seyferts(NGC 3783, NGC 3516, and Fairall 9).
The “secure” events span durations from less than one
day (for NGC 5506) to 1.5 years (for Mkn 348). The
clouds are modeled to have column densities spanning
4 − 26 × 1022 cm−2 ; we did not detect any full-covering
Compton-thick clouds. Assuming a Galactic gas/dust
ratio, these X-ray columns imply dust with V-band ex-

4. Resolving clouds’ density profiles
When adequate time-resolved data are present, we canobtain information on a given cloud’s density profile in
the transverse direction, and recent results are beginning
to reveal a diversity in cloud profile shapes.

10
5

NH (1022 cm−2)

NGC 3783

0

tinction AV = 22 − 144 mag., and V-band optical depths
τV of 20–132. These values are consistent with optical depths typically used in Clumpy modeling (Nenkova
et al. 2008) and derived by fits to IR SEDs by AlonsoHerrero et al. (2011) and Ramos Almeida et al. (2011).
To estimate the distance to a given cloud from the supermassive black hole rcl , we use the measured eclipse
duration and column density, combined with constraints
on the cloud ionization level. Specifically, we follow §4 of
Lamer et al. (2003) and assume that clouds are in Keplerian orbits and that each cloud has a uniform density.
Cloud diameter Dcl = NH /nH = vcl tD , where tD is themeasured√eclipse time and vcl is the transverse velocity,
equal to GMBH /rcl . Using the definition of ionization
parameter ξ, one obtains (see Eqn. 3 of Lamer et al. 200
1/5 2/5 2/5 −2/53): rcl = 4 × 1016 M7 L42 tD NH,22 ξ −2/5 cm, where
M7 = MBH /106 M⊙ , L42 = Lion /(1042 erg s−1 ), NH,22 =NH /(1022 cm−2 ), and tD is in units of days.
Our constraints on the ionization parameters for ourclouds are poor. Given the column densities of our clouds
and the energy resolution of the PCA, we can safely ruleout values of log(ξ) above ∼ 2. We therefore calculatedistances assuming log(ξ) = −1, 0, or +1. Uncertainty
on rcl is thus dominated by uncertainty on ξ.
We obtain best-estimate values of rcl that are typicallytens to hundreds of light-days from the central engine,and lie in the range 1 − 50 × 104 Rg (0.3 − 140 × 104 Rg
accounting for uncertainties).
The clouds in 7/8 objects (all except NGC 5506) areconsistent with the dust sublimation zone considering
uncertainties on rcl , i.e., in the outer BLR and/or innerdusty torus. However, this “clustering” may be in partassociated with our observational bias to select eclipses
with events of ∼tens of days, as per our selection function. In Cen A, the clouds are inferred to be consistent
with residing entirely in the dusty zone. In contrast,
NGC 5506 has the lowest value of rcl /Rd ; this cloud islikely the least dusty of the secure events in our sample,
and is likely commensurate with that source’s BLR.
The average cloud diameter was ∼ 5 light-hours(roughly the Sun–Neptune distance). In addition, theinferred diameters and locations for these (full-covering)
clouds imply upper limits to the size of the X-ray
continuum-emitting region. These limits are as low as
2Rg for NGC 5506 and 45−65Rg for NGC 3783, Mkn 79,
and Mkn 509. The average cloud mass was 10−5.2 M⊙ ,and the average number density was 108.5 cm−3 .

5

10

15

20

25

30

35

Days since MJD 54550

✸✂

✁✇✐❢t
④ ❙✉③❛❦✉

✸✄
✷✂

✎❪
✏
✎✎

♠❝ ☎✆
✍

✌❬ ✶✝
❍
◆
✞✄
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✵

✺✹✼✼✵

✺✹✼✽✵
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❞✟✠❡ ✭▼♦❞✡☞❡❞ ❏☛❧✡✟♥ ❉✟②✮

Fig. 1. The community is starting to amass a variety in behavior inclouds’ column density profiles as traced by NH (t), when adequate
sampling and signal-to-noise allow. (Top panel) — From MKN14:
an 18-day long eclipse event in NGC 3783 as traced by RXTE,
revealing two column density peaks. (Bottom panel) — From
Beuchert et al. (2015): a five-week Suzaku/Swift campaign on
NGC 3227 in 2008 caught the source obsured by an ionized cloudwith an irregular density profile.

For example, the 2010–11 eclipse in Cen A observed byRivers et al. (2011) had a NH profile that was symmetricin time and centrally-peaked; Rivers et al. (2011) foundthat a centrally-peaked number density fit better thana uniform number-density sphere. Lamer et al. (2003)
and Sanfrutos et al. (2013) also observed symmetric NH
profiles in the 2000–1 eclipse of NGC 3227 and an eclipse
in SWIFT J2127.4+5654, respectively.
However, many other eclipse profiles reveal embedded overdense/underdense regions: a double-peaked NH
profile in NGC 3783 observed by MKN14, a highlyvariable profile in a NGC 3227 event during a fiveweek Suzaku/Swift campaign measured by Beuchert etal. (2015) (see Fig. 1), and “comet” and “anti-comet”
shapes observed in NGC 1365 by Maiolino et al. (2010)
and Rivers et al. (2015).
These profiles may be clues as to how clouds areformed and sculpted, e.g., the torus may be a dynamically active place, with eclipsing clouds possibly being due to MHD winds launched upwards from thedisk (Fukumura et al. 2010; Czerny & Hryniewicz 2011)
and/or compressed by poloidal magnetic fields (Emmering et al. 1992).

5. On non-variable absorption in type IIs
We find evidence in eight type II Seyferts for a baseline
level of X-ray absorption that is constant in time overtimescales from 0.6 to 8.4 years; the observed hardness
ratios place limits on ∆NH ∼ 0.6 − 9 × 1022 cm−2 (seeFig. 2). In the context of clumpy-torus models explaining all the observed absorption, one would require that alarge number of very low-column density (≪ 1022 cm−2 )clouds along the line of sight, with the total number ofclouds along thre line of sight remaining roughly constant within each object. Alternately, the constant levelof absorption can be explained by absorbing structuresassociated with the host galaxy, e.g., dust lanes (Gould
et al. 2012). Another possibility is a medium of nonclumpy, highly homogeneous gas close to the black hole,such as a homogeneous intercloud medium.
However, in Cen A, MKN14 detected an 80-day “antieclipse”: the baseline level of absorption temporarily
dipped by ∼ 14% then recovered. This event suggeststhat material close to the black hole is not highly homogeneous, and that we witnessed a transit across the lineof sight by a relatively underdense region.
6. Instantaneous probabilities to witness an eclipse
We derived the probability to catch a type I/II source undergoing an eclipse event that has any duration between
0.2 d and 16 yr, taking into account the highly inhomogeneous sampling in our X-ray observations. That is, weestimated the probability to instantaneously observe asource undergoing an eclipse event due only to a cloud
passage through the line of sight, and independent oflong-term constant absorption, e.g., associated with gas
in the host galaxy.
For type Is, it is 0.006 (error range: 0.003–0.166); fortype IIs, 0.110 (0.039–0.571). Our uncertainties are conservative, as they take into account our selection function, candidate eclipse events in addition to the secureones, uncertainties in the observed durations, and uncertainties in the contributions of individual objects sampling patterns to our total sensitivity function; we useda Monte Carlo ”bootstrap” method to estimate uncertainties.
Although subject to low number statistics, our observations hint at diﬀerences in the distributions of observed
eclipse event durations and probabilities for type I andII objects. In addition, assuming that all type Is or
IIs have a common cloud distribution morphology, thenthese instantaneous probability values can be comparedto Clumpy theoretical predictions that assume certain
cloud distribution parameters (Clumpy parameters inclination angle, angular distibution of clouds, and number of clouds along an equatorial ray) to provide constraints on these parameters.
However, as a caveat, these probabilities were derived

with a sample that had some biases due to highly inhomogeneous X-ray sampling, and due to type Is’ being
sampled ∼ 4 − 5 times more often than type IIs. We
were also limited in sensitivity to changes in column density no lower than ∼ 1022 cm−2 . In addition, we were
not highly sensitive to absorption by highly-ionized or
partial covering gas; our PCA spectra were sensitive toionization levels up to log ξ ∼ 1 − 2. When one considers
the full range of possible cloud properties, the resulting
probabilities will almost certainly be higher.
7. Preliminary results from Nikutta et al., in prep: additional constraints on cloud properties
We also present a brief overview of results from our forthcoming paper in which we derive additional constraints
on cloud properties.
The “long-term event” clouds in MKN14 inferred toreside in the outer BLR/inner dusty torus and the“short-term event” clouds reported by Risaliti et al.
(2007), Risaliti et al. (2009), Maiolino et al. (2010), Sanfrutos et al. (2013), and Turner et al. (2008) and inferred
to reside in the BLR are all roughly consistent with acommon angular size as seen from the central source,∼ 1.4 arcmin. That is, cloud physical diameters are consistent with increasing linearly with linear distance fromthe central black hole. This fact may provide a hint atcloud formation mechanisms: if clouds are formed fromthe disk, for example, various cloud fragmentation mechanisms e.g., Amaro-Seoane & Chen (2014), may be applicable.
This small average angular size also suggests thatin order for clumpy-torus models to explain the total
observed median line of sight absorption in type I/II
Seyferts (Ramos Almeida et al. 2011), a total of 4 × 107(type Is) or 8 × 107 (type IIs) clouds may exist across the
BLR + dusty torus (assuming no cloud overlap). These
values assume that clouds only account for all absorption; the presence of an intercloud medium or absorption due to host galaxy dust lanes along the line of sightwould of course yield lower cloud numbers.
We also infer that the 12 secure clouds events from
MKN14 do not possess suﬃcient self-gravitational resistance to tidal shearing. A cloud in orbit will be able
to withstand gravitational torque only if its density isgreater than the “ambient” local density of the black holemass spread out evenly over the sphere of the cloud’s
orbit. We use the estimates of cloud number densityfrom MKN14, and find that clouds are underdense byfactors of 10–1000 to resist tidal shearing. The ambient pressure from an inter-cloud medium could suﬃce toconfine clouds (Netzer 2013). Alternatively, an ambient
magnetic field can confine clouds, as suggested by Rees
(1987) for BLR clouds (although the argument also applies to torus clouds); we estimate that the minimum re-quired magnetic field strength is only a few milli-Gauss.
8. Future prospects
While the RXTE sample represents the best current opportunity to explore long-term eclipses and clouds inthe outer BLR/dusty torus, we were limited to exploring neutral or near-neutral, full-covering or near-fullcovering (covering fraction ≳ 90%) clouds.
To eﬀectively explore larger regions of [ NH , covering fraction, ξ, ∆t ] parameter space in the future,
our idealized ”X-ray monitoring wish list” for obtaining additional constraints from X-ray eclipses thus includes: •Sustained monitoring with evenly-spaced sampling, down to timescales of hours, with consistentsource-to-source coverage to mitigate observing biases
as a function of time. •Soft X-ray coverage to accesslower column densities and lower values of ∆NH compared to PCA, and to access constraints on cloud ionization parameter. •Access to lower 2–10 keV fluxes
and a larger number of type IIs, as they are expected
to harbor a higher number of line of sight clouds iforientation is the dominant factor in type I/II unification. In the context of clouds being produced in adisk wind, however, BLR/torus cloud production may
be hampered towards low luminosities (Elitzur & Ho2009), and cloud numbers as a function of luminosity
across the BLR and dusty torus must be tested. We alsoneed to test whether “baseline” levels of absorption are
truly constant, indicating either host galaxy absorption
or a highly-homogeneous intercloud medium, or variable,suggesting transiting over- or under-dense regions close
to the AGN. •We must span a wider range of luminosities than in MKN14 to test if covering factor evolves withluminosity, as implied by multiple high-redshift studies(Ueda et al. 2014). •At least CCD-quality spectral monitoring to fully deconvolve variations in NH and Γ, andconfirm partial-covering events. •Ideally, if optical/UV
monitoring is in place simultaneous to X-ray monitoring,such monitoring can tell us whether the cloud is dusty.
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NGC 5506
1998

1999

2000

2001

2002

2003

2004

5 10 15 20

1997

2
1
1.5
1
0.5

F7−10//F2−4

F2−4/µ F7−10/µ

3

F2−10

1996

5.05×104

5.1×104

5.15×104

5.2×104

5.25×104

5.3×104

Fig. 2. From MKN14: RXTE -PCA light curve of the Sy 1.9 NGC 5506, showing light curves for the 2–10, 2–4, 7–10 keV bands, and the
7–10/2–4 keV hardness ratio. The hardness ratio is consistent with a baseline level of absorption that is constant down to ∆NH ∼ 1×1022
cm−2 for a duration of eight years; the exception is the short-term cloud transit in 2000. (The mild shift in average harness ratio in 1999
is instrumental, and not intrinsic to the source.)